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SIOUX FALLS, S.D. (AP) -- The anticipated retirement announcement from South Dakota Sen. Tim Johnson gives Republicans one of their best chances of picking up a seat in their quest to regain control of the Senate, as the veteran moderate Democrat steps aside.

Johnson, chairman of the Senate Banking Committee, is expected to announce this afternoon that he will not seek a fourth term in the Senate next year.

The fifth Senate Democrat to call it quits, Johnson was facing a potentially difficult challenge from popular Republican Gov. Mike Rounds and still coping with the constraints of a 2006 brain hemorrhage that left his speech impaired and limited his mobility. The absence of the well-funded former congressman who has never lost an election in this GOP-trending state pushed the race to the top of the priority list, Republican strategists said.

"I believe South Dakota moves into the top slot as the most likely Republican pickup," said Greg Strimple, a Republican pollster and past consultant to the National Republican Senatorial Committee.

Johnson was scheduled to speak today at the University of South Dakota in Vermillion.

For weeks, Johnson's longtime aides and allies have suggested the 66-year-old would step down in 2014, despite his return to the Senate after the life-threatening hemorrhage. His recovery has been significant, though his speech remains impaired and he sometimes uses a motorized scooter.

Johnson's re-election in 2008 after the brain injury sealed his reputation for resilience. But he had already established his reputation as a loyal Democrat with an independent streak that made him a formidable candidate. He won re-election to the Senate against the popular Republican U.S. Rep. John Thune, now South Dakota's junior senator, after voting against the resolution to authorize the use of military force in Iraq and despite campaigning for Thune by Republican President George W. Bush.

Johnson has sided with Democrats on key issues such as the 2010 Affordable Care Act. He also has been an environmental advocate. But he has supported the Keystone XL oil pipeline, which is mapped to cross South Dakota and fiercely opposed by environmentalists.

He also has $1.2 million in his re-election campaign account, a healthy nest egg for a state where television advertising is relatively inexpensive. He retains a robust fundraising network, thanks to his deep-pocketed committee connections.

Despite those advantages, Johnson joins Democratic Sens. Carl Levin of Michigan, Tom Harkin of Iowa, Jay Rockefeller of West Virginia, and Frank Lautenberg of New Jersey as seasoned and influential Democrats departing the chamber, where Republicans need to gain six seats to take control. Among those states, West Virginia was carried by Republican presidential candidate Mitt Romney last year.

Two Republican senators have announced their retirements, both in Republican-leaning states, Georgia and Nebraska.

South Dakota, a reliably independent state just a decade ago, has trended sharply Republican in the past decade. Where Democrats once boasted two influential Democratic senators, Republicans now control the Legislature, the governorship, the lone U.S. House seat and the other Senate seat.

But Democrats dismissed the notion that Johnson's retirement opens the door for a GOP senator. In last November's election, some Republican Senate candidates who appeared to be the heavy favorites ended up losing to Democratic rivals — including Rick Berg, who lost to Heidi Heitkamp in neighboring North Dakota.

"I reject the idea that somehow the Republicans have a lock on this state," South Dakota Democratic Party Chairman Ben Nesselhuf said. "By no means is this an impossible task, or even improbable."

Devotees say Johnson's personality — reserved and contemplative — has been key to the respect and influence he has amassed.

Bernie Hunhoff, minority leader in the South Dakota House, described Johnson as a pioneering advocate for women's and children's issues during his early days in the state Legislature. Johnson set the standard for Democrats by staying true to progressive principles while also reaching out across the electorate in a politically diverse state, said Hunhoff.

"He's more progressive than the average voter, but they obviously set that aside because they liked what they saw," Hunhoff said. "I think you'd describe Tim as a typical South Dakotan — quiet, stoic, practical — a good neighbor."

Despite contentious congressional and campaign battles, Johnson has remained "a gentleman," known more for thinking than for talking, former South Dakota Democratic Party Chairwoman Judy Olson Duhamel said. "He's earned the respect of everybody he's ever worked with or for," she said.

Attempts by The Associated Press to reach Johnson through his top Senate staff were unsuccessful. Johnson aides would not confirm his retirement but said the senator would announce his political plans Tuesday in Vermillion.

Aware that Johnson might decide to retire, Democrats in South Dakota and nationally have discussed possible successors on the ticket, including Johnson's son Brendan, South Dakota's U.S. attorney. The younger Johnson said Monday in an interview that he was unaware of his father's decision and declined to discuss whether he would seek the office.

Former U.S. Rep. Stephanie Herseth Sandlin, a contemporary of Brendan Johnson and another heir to a South Dakota Democratic legacy, also is looking at running. A granddaughter of former South Dakota Gov. Ralph Herseth, she served six years in the U.S. House before being defeated for re-election in 2010.

Brendan Johnson, appointed U.S. attorney in 2009, has never held elected office and faced questions about his father's involvement in the confirmation process. Assets for the younger Johnson include his father's advisers and donor base.

Herseth Sandlin also has an intact network and following in South Dakota, but she could face problems in a potential primary with Johnson. She opposed the 2010 Affordable Care Act, a position that is out of step with a majority of party loyalists.

Brokaw reported from Pierre, S.D.; David Espo contributed from Washington.